Semiconductor device capable of avoiding latchup breakdown resulting from negative variation of floating offset voltage

ABSTRACT

A semiconductor device is provided which is capable of avoiding malfunction and latchup breakdown resulting from negative variation of high-voltage-side floating offset voltage (VS). In the upper surface of an n-type impurity region, a p + -type impurity region is formed between an NMOS and a PMOS and in contact with a p-type well. An electrode resides on the p + -type impurity region and the electrode is connected to a high-voltage-side floating offset voltage (VS). The p + -type impurity region has a higher impurity concentration than the p-type well and is shallower than the p-type well. Between the p + -type impurity region and the PMOS, an n + -type impurity region is formed in the upper surface of the n-type impurity region. An electrode resides on the n + -type impurity region and the electrode is connected to a high-voltage-side floating supply absolute voltage (VB).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices, and particularlyto a power device driving device for driving power devices, e.g.inverters.

2. Description of the Background Art

A power device includes series-connected first and second N-channelinsulated-gate bipolar transistors (IGBTs) and a power device drivingdevice. The first IGBT has its collector electrode connected to a mainpower-supply and the second IGBT has its emitter electrode connected toa ground potential. The emitter electrode of the first IGBT and thecollector electrode of the second IGBT are connected to a load.Free-wheeling diodes are connected in inverse-parallel respectively tothe first and second IGBTs in order to protect the first and secondIGBTs from counterelectromotive force due to the load.

The power device driving device includes a high-voltage-side drivingportion for controlling the first IGBT and a low-voltage-side drivingportion for controlling the second IGBT. The power device driving devicehas a VS terminal connected to the emitter electrode of the first IGBT,a VB terminal connected to the emitter electrode of the first IGBTthrough a capacitor, an HO terminal connected to the control electrodeof the first IGBT, a COM terminal connected to the emitter electrode ofthe second IGBT, a VCC terminal connected to the emitter electrode ofthe second IGB0T through a capacitor, an LO terminal connected to thecontrol electrode of the second IGBT, and a GND terminal. VS denotes ahigh-voltage-side floating offset voltage that serves as a referencepotential for the high-voltage-side driving portion. VB is ahigh-voltage-side floating supply absolute voltage serving as apower-supply for the high-voltage-side driving portion, which issupplied from a high-voltage-side floating power-supply. HO is ahigh-voltage-side driving signal output by the high-voltage-side drivingportion. COM is common ground. VCC is a low-voltage-side fixed supplyvoltage serving as a power-supply for the low-voltage-side drivingportion, which is supplied from a low-voltage-side fixed supplypower-supply. LO is a low-voltage-side driving signal output by thelow-voltage-side driving portion. GND is a ground potential.

Now, the conventional power device driving device, specifically thehigh-voltage-side driving portion, is described.

The high-voltage-side driving portion includes a CMOS circuit havingPMOS and NMOS transistors. The PMOS transistor has its source electrodeconnected to the VB terminal, the NMOS transistor has its sourceelectrode connected to the VS terminal, and the PMOS and NMOStransistors have their respective drain electrodes connected to the HOterminal.

Next, the structure of the conventional semiconductor device having theCMOS circuit is described. The semiconductor device has a p⁻-typesilicon substrate, an n-type impurity region formed in the upper surfaceof the p⁻-type silicon substrate, a p-type well formed in the uppersurface of the n-type impurity region, n-type source and drain regionsof the NMOS transistor that are formed in the upper surface of thep-type well, p-type source and drain regions of the PMOS transistor thatare formed in the upper surface of the n-type impurity region, and ap⁺-type isolation region formed in the upper surface of the p⁻-typesilicon substrate and in contact with the n-type impurity region.

A channel formation region is defined between the source and drainregions of the NMOS transistor and a gate electrode of the NMOStransistor resides on the channel formation region with a gateinsulating film disposed between them. Likewise, a channel formationregion is defined between the source and drain regions of the PMOStransistor and a gate electrode of the PMOS transistor resides on thechannel formation region with a gate insulating film disposed betweenthem. The source region of the NMOS transistor is connected to the VSterminal and the source region of the PMOS transistor is connected tothe VB terminal. The drain regions of the NMOS and PMOS transistors areconnected to the HO terminal in common.

Techniques about semiconductor devices having CMOS circuits aredisclosed in Japanese Patent Application Laid-Open Nos. 11-68053 (1999),62-120063 (1987), 60-74560 (1985), and 5-152523 (1993).

In the conventional power device and power device driving device, thehigh-voltage-side floating offset voltage VS may vary to a negativevoltage lower than the common ground COM during regenerative periods(i.e. periods in which free-wheeling diodes turn on due to thecounterelectromotive force from the load). The negative variation of thehigh-voltage-side floating offset voltage VS is transmitted to thehigh-voltage-side floating supply absolute voltage VB through thecapacitor and then the potential of the high-voltage-side floatingsupply absolute voltage VB, too, makes a negative variation.

The negative variation of the high-voltage-side floating supply absolutevoltage VB is transmitted to the n-type impurity region. As a result,parasitic diodes between the p⁺-type isolation region and n-typeimpurity region and parasitic diodes between the p⁻-type siliconsubstrate and n-type impurity region, which are normally reverse-biased,turn on, which causes current to flow into the n-type impurity region.

Then, in the conventional semiconductor device, the current flowing intothe n-type impurity region due to the turning-on of the parasiticdiodes, may cause the high-voltage-side driving signal output HO to belogically inverted (malfunction), or may cause a parasitic thyristor tolatch up to cause excessive current flow to the CMOS circuit, which maydamage the circuitry or parts (latchup breakdown: for more details,refer to Japanese Patent Application Laid-Open No. 2002-252333 by thesame applicant).

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor devicecapable of avoiding malfunction and latchup breakdown resulting fromnegative variation of a floating offset voltage.

A first aspect of the invention is related to a semiconductor device fordriving a switching device having a first electrode, a second electrode,and a control electrode. According to the first aspect of the invention,the semiconductor device includes a first terminal, a second terminal, asemiconductor substrate of a first conductivity type, a first impurityregion of a second conductivity type, a second impurity region of thefirst conductivity type, a first transistor, a second transistor, and atleast one of a third impurity region of the first conductivity type anda fourth impurity region of the second conductivity type. The firstterminal is connected to the first electrode. The second terminal isconnected to the first electrode through a capacitive element. The firstimpurity region is formed in a main surface of the semiconductorsubstrate. The second impurity region is formed in a main surface of thefirst impurity region. The first transistor has a source/drain region ofthe second conductivity type that is formed in a main surface of thesecond impurity region and connected to the first terminal. The secondtransistor has a source/drain region of the first conductivity type thatis formed in the main surface of the first impurity region and connectedto the second terminal. The third impurity region is formed in the mainsurface of the first impurity region and connected to the firstterminal. The fourth impurity region is formed in the main surface ofthe first impurity region and connected to the second terminal.

It is thus possible to avoid latchup breakdown resulting from negativevariation of the floating offset voltage.

A second aspect of the invention is related to a semiconductor devicefor driving a switching device having a first electrode, a secondelectrode, and a control electrode. According to the second aspect, thesemiconductor device includes a first terminal, a second terminal, asemiconductor substrate of a first conductivity type, a first impurityregion of a second conductivity type, a second impurity region of thefirst conductivity type, a first transistor, a second transistor, and atleast one of a third impurity region of the first conductivity type anda fourth impurity region of the first conductivity type. The firstterminal is connected to the first electrode. The second terminal isconnected to the first electrode through a capacitive element. The firstimpurity region is formed in a main surface of the semiconductorsubstrate. The second impurity region is formed in a main surface of thefirst impurity region. The first transistor has a source/drain region ofthe second conductivity type that is formed in a main surface of thesecond impurity region and connected to the first terminal. The secondtransistor has a source/drain region of the first conductivity type thatis formed in the main surface of the first impurity region and connectedto the second terminal. The third impurity region is formed in the mainsurface of the first impurity region and connected to the firstterminal. The fourth impurity region is formed in the main surface ofthe first impurity region and connected to the second terminal.

It is thus possible to avoid latchup breakdown resulting from negativevariation of the floating offset voltage.

A third aspect of the invention is related to a semiconductor device fordriving a switching device having a first electrode, a second electrode,and a control electrode. According to the third aspect, thesemiconductor device includes a first terminal, a second terminal, asemiconductor substrate of a first conductivity type, a first impurityregion of a second conductivity type, a second impurity region of thefirst conductivity type, a first transistor, a second transistor, athird impurity region of the first conductivity type, a trench, and afourth impurity region of the first conductivity type. The firstterminal is connected to the first electrode. The second terminal isconnected to the first electrode through a capacitive element. The firstimpurity region is formed in a main surface of the semiconductorsubstrate. The second impurity region is formed in a main surface of thefirst impurity region. The first transistor has a source/drain region ofthe second conductivity type that is formed in a main surface of thesecond impurity region and connected to the first terminal. The secondtransistor has a source/drain region of the first conductivity type thatis formed in the main surface of the first impurity region and connectedto the second terminal. The third impurity region is formed in the mainsurface of the first impurity region and connected to the first orsecond terminal. The trench is formed in the main surface of the firstimpurity region and extends through the third impurity region. Thefourth impurity region is formed in the first impurity region in aportion that defines wall surfaces of the trench and connected to thefirst or second terminal.

It is thus possible to avoid latchup breakdown resulting from negativevariation of the floating offset voltage.

A fourth aspect of the invention is related to a semiconductor devicefor driving a switching device having a first electrode, a secondelectrode, and a control electrode. According to the fourth aspect ofthe invention, the semiconductor device includes a first terminal, asecond terminal, a semiconductor substrate of a first conductivity type,a first impurity region of a second conductivity type, a second impurityregion of the first conductivity type, a first transistor, a secondtransistor, and at least one of a combination of a third impurity regionof the first conductivity type and a fourth impurity region of thesecond conductivity type, and a combination of a fifth impurity regionof the second conductivity and a sixth impurity region of the firstconductivity. The first terminal is connected to the first electrode.The second terminal is connected to the first electrode through acapacitive element. The first impurity region is formed in a mainsurface of the semiconductor substrate. The second impurity region isformed in a main surface of the first impurity region. The firsttransistor has a source/drain region of the second conductivity typethat is formed in a main surface of the second impurity region andconnected to the first terminal. The second transistor has asource/drain region of the first conductivity type that is formed in themain surface of the first impurity region and connected to the secondterminal. The third impurity region is formed in the main surface of thesecond impurity region and connected to the first terminal. The fourthimpurity region is formed in the main surface of the second impurityregion and in contact with the third impurity region and is connected tothe first terminal. The fifth impurity region is formed in the mainsurface of the first impurity region and connected to the secondterminal. The sixth impurity region is formed in the main surface of thefirst impurity region and in contact with the fifth impurity region andis connected to the second terminal.

It is thus possible to avoid latchup breakdown resulting from negativevariation of the floating offset voltage.

A fifth aspect of the invention is related to a semiconductor device fordriving a switching device having a first electrode, a second electrode,and a control electrode. According to the fifth aspect of the invention,the semiconductor device includes a first terminal, a second terminal, asemiconductor substrate of a first conductivity type, a first impurityregion of a second conductivity type, a second impurity region of thefirst conductivity type, a first transistor, a second transistor, athird impurity region of the first conductivity type, a trench, a fourthimpurity region of the first conductivity type, a fifth impurity regionof the second conductivity type, and a floating electrode. The firstterminal is connected to the first electrode. The second terminal isconnected to the first electrode through a capacitive element. The firstimpurity region is formed in a main surface of the semiconductorsubstrate. The second impurity region is formed in a main surface of thefirst impurity region. The first transistor has a source/drain region ofthe second conductivity type that is formed in a main surface of thesecond impurity region and connected to the first terminal. The secondtransistor has a source/drain region of the first conductivity type thatis formed in the main surface of the first impurity region and connectedto the second terminal. The third impurity region is formed in the mainsurface of the first impurity region. The trench is formed in the mainsurface of the first impurity region and extends through the thirdimpurity region. The fourth impurity region is formed in the firstimpurity region in a portion that defines wall surfaces of the trench.The fifth impurity region is formed in the main surface of the firstimpurity region and in contact with the third impurity region. Thefloating electrode is formed in the main surface of the first impurityregion and in contact with the third to fifth impurity regions.

It is thus possible to avoid latchup breakdown resulting from negativevariation of the floating offset voltage.

A sixth aspect of the invention is related to a semiconductor device fordriving a switching device having a first electrode, a second electrode,and a control electrode. According to the sixth aspect of the invention,the semiconductor device includes a first terminal, a second terminal, afirst impurity region of a first conductivity type, a second impurityregion of a second conductivity type, a first transistor, a secondtransistor, and a third impurity region of the second conductivity type.The first terminal is connected to the first electrode. The secondterminal is connected to the first electrode through a capacitiveelement. The second impurity region is formed in a main surface of thefirst impurity region. The first transistor has a source/drain region ofthe first conductivity type that is formed in a main surface of thesecond impurity region and connected to the first terminal. The secondtransistor has a source/drain region of the second conductivity typethat is formed in the main surface of the first impurity region andconnected to the second terminal. The third impurity region is formed inthe main surface of the first impurity region and connected to the firstterminal.

It is thus possible to avoid latchup breakdown resulting from negativevariation of the floating offset voltage.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram used to describe the structure of a powerdevice and a power device driving device;

FIG. 2 is a circuit diagram showing a main part of the high-voltage-sidedriving portion;

FIG. 3 is a schematic plan showing the layout on a high-voltage islandin the power device driving device;

FIGS. 4 and 5 are cross-sectional views showing the structure of asemiconductor device according to a first preferred embodiment of theinvention;

FIGS. 6 and 7 are top views schematically showing examples of the layoutof the n⁺-type impurity region and p⁺-type impurity region;

FIG. 8 is a cross-sectional view used to describe effects of thesemiconductor device of the first preferred embodiment of the invention;

FIG. 9 is a cross-sectional view showing the structure of asemiconductor device according to a second preferred embodiment of theinvention;

FIG. 10 is a cross-sectional view showing the structure of asemiconductor device according to a third preferred embodiment of theinvention;

FIG. 11 is a cross-sectional view showing the structure of asemiconductor device according to a fourth preferred embodiment of theinvention;

FIG. 12 is a top view schematically showing the layout of the p⁺-typeimpurity region and p⁺-type impurity region;

FIG. 13 is a cross-sectional view showing the structure of asemiconductor device according to a fifth preferred embodiment of theinvention;

FIG. 14 is a cross-sectional view showing the structure of asemiconductor device according to a sixth preferred embodiment of theinvention;

FIG. 15 is a cross-sectional view showing the structure of asemiconductor device according to a seventh preferred embodiment of theinvention; and

FIG. 16 is a cross-sectional view showing the structure of asemiconductor device according to a tenth preferred embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Preferred Embodiment

FIG. 1 is a schematic diagram illustrating the configuration of a powerdevice and a power device driving device. N-channel insulated-gatebipolar transistors (IGBTs) Q1 and Q2, as power switching devices,switch a high voltage HV or a main power-supply. A load is connected toa node N30. Free-wheeling diodes D1 and D2 protect the IGBTs Q1 and Q2from counterelectromotive force due to the load connected to the nodeN30.

A power device driving device 100 drives IGBTs Q1 and Q2; the powerdevice driving device 100 operates in accordance with ahigh-voltage-side control input HIN for controlling IGBT Q1 and alow-voltage-side control input LIN for controlling IGBT Q2. The powerdevice driving device 100 includes a high-voltage-side driving portion101 for driving IGBT Q1, a low-voltage-side driving portion 102 fordriving IGBT Q2, and a control input processing portion 103.

Now, when IGBTs Q1 and Q2 simultaneously turn on, for example, a throughcurrent flows in IGBTs Q1 and Q2 and then no current flows to the load,which is an undesirable condition. The control input processing portion103, receiving the control inputs HIN and LIN, controls thehigh-voltage-side driving portion 101 and low-voltage-side drivingportion 102 in order to, for example, prevent such an undesirablecondition.

Also, the power device driving device 100 has a VS terminal connected tothe emitter electrode of IGBT Q1, a VB terminal connected to the emitterelectrode of IGBT Q1 through a capacitor C1, an HO terminal connected tothe control electrode of IGBT Q1, a COM terminal connected to theemitter electrode of IGBT Q2, a VCC terminal connected through acapacitor C2 to the emitter electrode of IGBT Q2, an LO terminalconnected to the control electrode of IGBT Q2, and a GND terminal. VSdenotes a high-voltage-side floating offset voltage that serves as areference potential for the high-voltage-side driving portion 101. VB isa high-voltage-side floating supply absolute voltage serving as apower-supply for the high-voltage-side driving portion 101, which issupplied from a high-voltage-side floating power-supply not shown. HO isa high-voltage-side driving signal output by the high-voltage-sidedriving portion 101. COM is common ground. VCC is a low-voltage-sidefixed supply voltage serving as a power-supply for the low-voltage-sidedriving portion 102, which is supplied from a low-voltage-side fixedsupply power-supply not shown. LO is a low-voltage-side driving signaloutput by the low-voltage-side driving portion 102. GND is a groundpotential.

The capacitors C1 and C2 are provided to cause the power-supply voltagessupplied to the high-voltage-side driving portion 101 andlow-voltage-side driving portion 102 to follow potential variationscaused by the operation of the power device.

With this configuration, the main power-supply is switched by the powerdevice on the basis of the control inputs HIN and LIN.

Now, since the high-voltage-side driving portion 101 operates in apotential-floating condition with respect to the circuit groundpotential GND, the configuration has a level shift circuit fortransmitting the driving signal to the high-voltage-side circuit.

FIG. 2 is a circuit diagram showing the main part of thehigh-voltage-side driving portion 101 in the power device driving device100. A high-breakdown-voltage MOS 11, a switching element, serves as thelevel shift circuit mentioned above. A CMOS circuit (hereinafterreferred to as CMOS) 12, a switching component for the high-voltage-sidedriving signal output, includes a PMOS transistor (hereinafter referredto as PMOS) 15 and an NMOS transistor (hereinafter referred to as NMOS)14; the CMOS circuit 12 outputs the high-voltage-side driving signal HO.A level shift resistor 13, for setting the gate potential for the CMOS12, serves as pull-up resistance.

The high-breakdown-voltage MOS 11 switches the CMOS 12 in accordancewith the high-voltage-side control input HIN. The CMOS 12 switches thevoltage between the high-voltage-side floating supply absolute voltageVB and the high-voltage-side floating offset voltage VS and outputs adriving signal to the high-voltage-side driving signal output HO,thereby driving the high-voltage-side switching element of the externalpower device.

In the description below, the CMOS 12 and level shift resistor 13 arereferred to together as a high-voltage-side driving circuit.

FIG. 3 is a schematic plan showing the layout of a high-voltage islandin the power device driving device 100. The high-voltage-side drivingcircuit, including CMOS 12 and level shift resistor 13, is formed withina region R2 that is called a high-voltage island, with thehigh-breakdown-voltage MOS 11 formed in a region R1. Aluminum lines 16and 17, for ground potential GND, surround and shield them.

FIGS. 4 and 5 are cross-sectional views showing the structure of thesemiconductor device of a first preferred embodiment of the invention,which are the cross-sections taken respectively along lines IV-IV andV-V in FIG. 3. Referring to FIG. 4, in the upper surface of a p⁻-typesilicon substrate 21, a p⁺-type isolation region 22, an n⁻-type impurityregion 23, and n-type impurity regions 24 and 28 are formed. A p-typewell 29 is formed in the upper surface of the n-type impurity region 28.The p⁺-type isolation region 22 reaches the p⁻-type silicon substrate21. An electrode 34 resides on the p⁺-type isolation region 22 and thepotential at the p⁻-type silicon substrate 21 is the lowest in thecircuit (GND or COM potential). A p-type well 25 is formed under ann⁺-type source region 11S of the high-breakdown-voltage MOS 11; thep-type well 25 extends under the gate electrode 11G with a gateinsulating film disposed between them and forms the channel region ofthe high-breakdown-voltage MOS 11. In the p-type well 25, a p⁺-typeimpurity region 26 and the n⁺-type source region 11S are in contact withthe source electrode 35. Also, an n⁺-type drain region 11D is formed incontact with the drain electrode 36 of the high-breakdown-voltage MOS11.

The drain electrode 36 of the high-breakdown-voltage MOS 11 is connectedto the gate electrodes 15G and 14G of PMOS 15 and NMOS 14 of CMOS 12,and also connected to the source electrode 39 of PMOS 15 and thehigh-voltage-side floating supply absolute voltage VB through the levelshift resistor 13.

In the n-type impurity region 28 where CMOS 12 is formed, a p⁺-typesource region 15S and an n⁺-type impurity region 30 are formed incontact with the source electrode 39 of PMOS 15, and a p⁺-type drainregion 15D is formed in contact with the drain electrode 37. On theupper surface of the n-type impurity region 28, the gate electrode 15Gof PMOS 15 is formed with a gate insulating film disposed between them.An electrode 38 resides on the gate electrode 15G. NMOS 14 is formedwithin the p-type well 29, with an n⁺-type drain region 14D formed incontact with the drain electrode 44 of NMOS 14 and with an n⁺-typesource region 14S and a p⁺-type impurity region 31 in contact with thesource electrode 42. On the upper surface of the p-type well 29, thegate electrode 14G of the NMOS 14 resides with a gate insulating filmdisposed between them. An electrode 43 resides on the gate electrode14G.

In the upper surface of the n-type impurity region 28, a p⁺-typeimpurity region 33 is formed between the NMOS 14 and PMOS 15 and incontact with the p-type well 29. An electrode 41 resides on the p⁺-typeimpurity region 33 and the electrode 41 is connected to thehigh-voltage-side floating offset voltage VS. The p⁺-type impurityregion 33 has a higher impurity concentration than the p-type well 29,and the p⁺-type impurity region 33 is shallower than the p-type well 29.Between the p⁺-type impurity region 33 and PMOS 15, an n⁺-type impurityregion 32 is formed in the upper surface of the n-type impurity region28. An electrode 40 resides on the n⁺-type impurity region 32 and theelectrode 40 is connected to the high-voltage-side floating supplyabsolute voltage VB.

Referring to FIG. 5, in the upper surface of the p⁻-type siliconsubstrate 21, the p⁺-type isolation region 22, an n⁻-type impurityregion 50, and the n-type impurity region 28 are present. An electrode52 resides on the p⁺-type isolation region 22. An n⁺-type impurityregion 51 is formed in the upper surface of the n-type impurity region28 and an electrode 53 is formed on the n⁺-type impurity region 51. Theelectrode 53 is connected to the high-voltage-side floating supplyabsolute voltage VB.

As shown in FIG. 5, the p⁺-type source region 15S, n-type impurityregion 28, p-type well 29, and n⁺-type source region 14S form a lateralparasitic thyristor. The parasitic thyristor is formed of a pnp-typeparasitic bipolar transistor PBP1 and an npn-type parasitic bipolartransistor PBP2.

FIGS. 6 and 7 are top views schematically showing examples of the layoutof the n⁺-type impurity region 32 and p⁺-type impurity region 33. Thep⁺-type isolation region 22 is formed to squarely surround the regionsR1 and R2 as shown in FIG. 3. In the example of FIG. 6, the n⁺-typeimpurity region 32 and p⁺-type impurity region 33 are formed betweenopposite, or facing, sides of the NMOS 14 and PMOS 15. In the example ofFIG. 7, the p⁺-type impurity region 33 surrounds the p-type well 29 andthe n⁺-type impurity region 32 surrounds the p-type well 29 between thep⁺-type isolation region 22 and the p-type well 29. Furthermore, in thespecification, the terms “form to surround” includes not only the caseof forming to surround continuously, but also the case of forming tosurround intermittently. The example shown in FIG. 6 enables a smallerchip size than that of FIG. 7.

In the power device and power device driving device of the firstpreferred embodiment, suppose that, during a regenerative period, thehigh-voltage-side floating offset voltage VS has varied to a negativevoltage lower than the common ground COM. In this case, the negativevariation of the high-voltage-side floating offset voltage VS istransmitted to the high-voltage-side floating supply absolute voltage VBthrough the capacitor C1 shown in FIG. 1, which causes thehigh-voltage-side floating supply absolute voltage VB to make a negativepotential variation. Referring to FIGS. 4 and 5, the negative variationof the high-voltage-side floating supply absolute voltage VB istransmitted to the n⁻-type impurity regions 23 and 50 and the n-typeimpurity regions 24 and 28. As a result, the parasitic diodes PD1-PD4,which are normally reverse-biased, turn on, causing current to flow intothe n⁻-type impurity region 23 and the n-type impurity regions 24 and28.

However, according to the semiconductor device of the first preferredembodiment, electrons corresponding to the base current of the parasiticbipolar transistor PBP1 are absorbed by the n⁺-type impurity region 32that is at higher potential than the high-voltage-side floating offsetvoltage VS. The base current of the parasitic bipolar transistor PBP1 isthus suppressed and the parasitic thyristor does not turn on, so thatlatchup breakdown of the CMOS 12 can be avoided.

Also, holes corresponding to the base current of the parasitic bipolartransistor PBP2 are absorbed by the p⁺-type impurity region 33 that isat lower potential than the high-voltage-side floating supply absolutevoltage VB. The base current of the parasitic bipolar transistor PBP2 isthus suppressed and the parasitic thyristor does not turn on, so thatlatchup breakdown of the CMOS 12 can be avoided. Referring to FIG. 8,the hole current flowing from the p⁺-type isolation region 22 into then⁻-type impurity region 50 is absorbed by the p⁺-type impurity region 33before reaching the p-type well 29. Also, the hole current flowing intothe n-type impurity region 28 from the p⁻-type silicon substrate 21right under the p-type well 29 is attracted by electrons that areinjected from the n⁺-type impurity regions 30, 32 into the n-typeimpurity region 28, and is absorbed by the p⁺-type impurity region 33instead of flowing into the p-type well 29.

As shown in FIG. 5, the p⁺-type impurity region 33 is formed shallowerthan the p-type well 29. Therefore the electric field strength of thecurvature portion of the p⁺-type impurity region 33 is higher than theelectric field strength of the curvature portion of the p-type well 29.This allows the p⁺-type impurity region 33 to absorb larger holecurrent, making it possible to more effectively avoid latchup breakdownof the CMOS 12.

As stated earlier, the impurity concentration of the p⁺-type impurityregion 33 is higher than the impurity concentration of the p-type well29. Therefore the electric field strength of the curvature portion ofthe p⁺-type impurity region 33 is higher than the electric fieldstrength of the curvature portion of the p-type well 29. Furthermore,increasing the impurity concentration of the p⁺-type impurity region 33reduces the sheet resistance in the p⁺-type impurity region 33 and thecontact resistance between the p⁺-type impurity region 33 and electrode41. This allows the p⁺-type impurity region 33 to absorb larger holecurrent, making it possible to more effectively avoid latchup breakdownof the CMOS 12.

As shown in FIG. 5, the p⁺-type impurity region 33 is formed in contactwith the p-type well 29 that corresponds to the base layer of theparasitic bipolar transistor PBP2. Thus, the chip size can be smallerthan when a gap is present between the p⁺-type impurity region 33 andp-type well 29; furthermore, since the resistance of the base layer ofthe parasitic bipolar transistor PBP2 is reduced, the p⁺-type impurityregion 33 can absorb larger hole current, which more effectively avoidslatchup breakdown of the CMOS 12.

The largest part of the current that flows into the n-type impurityregion 28 because of negative variation of the high-voltage-sidefloating offset voltage VS is the current flowing from the p⁺-typeisolation region 22 into the n-type impurity region 28 through then⁻-type impurity region 50. Therefore, particularly when the p⁺-typeisolation region 22 is formed to surround the regions R1 and R2,forming, as shown in FIG. 7, the p-type impurity region 33 and n⁺-typeimpurity region 32 surrounding the p-type well 29 more effectivelyavoids latchup breakdown of the CMOS 12.

Though forming both n⁺-type impurity region 32 and p⁺-type impurityregion 33 is the most effective, forming just one of them is alsoeffective.

Second Preferred Embodiment

FIG. 9 is a cross-sectional view showing the structure of asemiconductor device according to the a second preferred embodiment ofthe invention: FIG. 9 is associated with FIG. 5. The n⁺-type impurityregion 32 and electrode 40 shown in FIG. 5 are replaced by a p⁺-typeimpurity region 55 and an electrode 56. The electrode 56 is connected tothe high-voltage-side floating supply absolute voltage VB. The impurityconcentration of the p⁺-type impurity region 55 is higher than theimpurity concentration of the p-type well 29 and the p⁺-type impurityregion 55 is shallower than the p-type well 29.

As in the example of FIG. 6, the p⁺-type impurity region 55 is formedbetween the opposite sides of the NMOS 14 and PMOS 15. Alternatively, asin the example of FIG. 7, the p⁺-type impurity region 55 is formedbetween the p⁺-type isolation region 22 and the p-type well 29 andsurrounds the p-type well 29.

With a negative variation of the high-voltage-side floating offsetvoltage VS, the high-voltage-side floating offset voltage VS and thehigh-voltage-side floating supply absolute voltage VB become lower thanthe potential of the p⁻-type silicon substrate 21 and p⁺-type isolationregion 22. Then, even if the parasitic diodes PD1-PD4 turn on andcurrent flows into the n-type impurity region 28, holes that correspondto the base current of the parasitic bipolar transistor PBP2 shown inFIG. 5 are absorbed by the p⁺-type impurity regions 55 and 33. The basecurrent of the parasitic bipolar transistor PBP2 is thus suppressed andthe parasitic thyristor does not turn on, which avoids latchup breakdownof the CMOS 12.

Furthermore, since the p⁺-type impurity region 55 replaces the n⁺-typeimpurity region 32, when the parasitic diodes PD1-PD4 shown in FIGS. 4and 5 turn on due to negative variation of the high-voltage-sidefloating offset voltage VS, no electrons are injected from the p⁺-typeimpurity region 55 into the n-type impurity region 28. This reduces thesupply of holes from the p⁻-type silicon substrate 21 and the p⁺-typeisolation region 22 and thus reduces the hole current flowing into then-type impurity region 28, whereby latchup breakdown of the CMOS 12 canbe more effectively prevented.

Although forming both p⁺-type impurity region 55 and p⁺-type impurityregion 33 is the most effective, forming just one of them is alsoeffective.

Third Preferred Embodiment

FIG. 10 is a cross-sectional view showing the structure of asemiconductor device according to a third preferred embodiment of theinvention: FIG. 10 is associated with FIG. 5. The n⁺-type impurityregion 32 and electrode 40 shown in FIG. 5 are replaced by a p⁺-typeimpurity region 57 and an electrode 58. The electrode 58 is connected tothe high-voltage-side floating offset voltage VS. The impurityconcentration of the p⁺-type impurity region 57 is higher than theimpurity concentration of the p-type well 29 and the p⁺-type impurityregion 57 is shallower than the p-type well 29.

As in the example of FIG. 6, the p⁺-type impurity region 57 is formedbetween the opposite sides of the NMOS 14 and PMOS 15. Alternatively, asin the example of FIG. 7, the p⁺-type impurity region 57 is formedbetween the p⁺-type isolation region 22 and the p-type well 29 andsurrounds the p-type well 29.

With a negative variation of the high-voltage-side floating offsetvoltage VS, the high-voltage-side floating offset voltage VS andhigh-voltage-side floating supply absolute voltage VB become lower thanthe potential of the p⁻-type silicon substrate 21 and p⁺-type isolationregion 22. Then, even if the parasitic diodes PD1-PD4 turn on andcurrent flows into the n-type impurity region 28, holes that correspondto the base current of the parasitic bipolar transistor PBP2 shown inFIG. 5 are absorbed by the p⁺-type impurity regions 57 and 33. The basecurrent of the parasitic bipolar transistor PBP2 is thus suppressed andthe parasitic thyristor does not turn on, which avoids latchup breakdownof the CMOS 12.

Furthermore, since the p⁺-type impurity regions 57 and 33 are bothconnected to the high-voltage-side floating offset voltage VS and are atthe same potential, the pnp-type parasitic bipolar transistor formed bythe p⁺-type impurity region 57, n-type impurity region 28, and p⁺-typeimpurity region 33 does not operate. Thus, malfunction of thehigh-voltage-side driving portion 101, resulting from operation of thisparasitic bipolar transistor, can be avoided.

Although forming both p⁺-type impurity region 57 and p⁺-type impurityregion 33 is the most effective, forming just one of them is alsoeffective.

Fourth Preferred Embodiment

FIG. 11 is a cross-sectional view showing the structure of asemiconductor device according to a fourth preferred embodiment of theinvention: FIG. 11 is associated with FIG. 5. The n⁺-type impurityregion 32 and electrode 40 shown in FIG. 5 are replaced by a p⁺-typeimpurity region 59 and an electrode 60. The electrode 60 is connected tothe high-voltage-side floating supply absolute voltage VB. The p⁺-typeimpurity region 59 is formed between the p⁺-type drain region 15D andthe n⁺-type impurity region 51.

FIG. 12 is a top view schematically showing the layout of the p⁺-typeimpurity region 59 and p⁺-type impurity region 33. The p⁺-type isolationregion 22 is formed to squarely surround the regions R1 and R2 as shownin FIG. 3. As shown in FIG. 12, the p⁺-type impurity region 33 is formedto surround the p-type well 29, and the p⁺-type impurity region 59 isformed between the p⁺-type isolation region 22 and the p-type well 29and PMOS 15 and extends in parallel to the p⁺-type isolation region 22,and thus surrounds the p-type well 29 and PMOS 15.

As mentioned earlier, the largest part of the current that flows intothe n-type impurity region 28 because of negative variation of thehigh-voltage-side floating offset voltage VS is the current flowing fromthe p⁺-type isolation region 22 into the n-type impurity region 28through the n⁻-type impurity region 50. Therefore, when, as shown inFIG. 12, the p⁺-type impurity region 59 is formed in parallel to thep⁺-type isolation region 22, the p⁺-type impurity region 59 canefficiently absorb holes corresponding to the base current of theparasitic bipolar transistor PBP2 before they flow into the p-type well29. The base current of the parasitic bipolar transistor PBP2 is thussuppressed and the parasitic thyristor does not turn on, whichefficiently avoids latchup breakdown of the CMOS 12.

Although forming both p⁺-type impurity region 59 and p⁺-type impurityregion 33 is the most effective, forming just one of them is alsoeffective.

Fifth Preferred Embodiment

FIG. 13 is a cross-sectional view showing the structure of asemiconductor device according to a fifth preferred embodiment: FIG. 13is associated with FIG. 9. A trench 63 is formed in the upper surface ofthe n-type impurity region 28 and extends through the p⁺-type impurityregion 55; the inside of the trench 63 is filled with silicon oxide film61 and polysilicon 62. Also, p⁺-type impurity region 64 is formed in theportion of the n-type impurity region 28 that defines wall surfaces ofthe trench 63. The p⁺-type impurity region 64 is in contact with thep⁺-type impurity region 55 and electrode 56.

As in the example of FIG. 6, the p⁺-type impurity regions 55 and 64 areformed between the opposite sides of the NMOS 14 and PMOS 15.Alternatively, as in the example of FIG. 7, the p⁺-type impurity regions55 and 64 are formed between the p⁺-type isolation region 22 and p-typewell 29 and surround the p-type well 29.

As compared with the structure shown in FIG. 9, forming the p⁺-typeimpurity region 64 on the wall surfaces of the trench 63 and in contactwith the p⁺-type impurity region 55 enlarges the effective area of thep⁺-type impurity region 55. Furthermore, the electric field strength atthe bottom of the trench 63 is higher than the electric field strengthat the curvature portion of the p-type well 29. Thus, as compared withthe structure shown in FIG. 9, the p⁺-type impurity regions 55 and 64can absorb more of the hole current flowing into the n-type impurityregion 28. It is thus possible to more efficiently avoid latchupbreakdown of the CMOS 12.

Moreover, since the formation of the p⁺-type impurity region 64 enhancesthe efficiency of hole current absorption, the area of the p⁺-typeimpurity region 55 can be smaller than in the structure of FIG. 9, whichallows the chip to be downsized.

In the example of FIG. 13, the trench 63 is deeper than the p-type well29. Forming the trench 63 deeper increases the area of the p⁺-typeimpurity region 64, thus enhancing the hole current absorptionefficiency.

However, when the trench 63 is deeper than 20 μm, then the breakdownvoltage is reduced (the voltage at which pn-junction breakdown occurswhen voltage is applied in the GND direction with VB and VS shorted).Accordingly, when it is necessary to also satisfy the breakdown voltagecharacteristic, the reduction in the breakdown voltage can be avoided bysetting the depth of the trench 63 at 20 μm or less.

Though forming both p⁺-type impurity regions 55, 64 and the p⁺-typeimpurity region 33 is the most effective, forming just one of them isalso effective. Also, though the electrode 56 is connected to thehigh-voltage-side floating supply absolute voltage VB in the structureof FIG. 13, the electrode 56 may be connected to the high-voltage-sidefloating offset voltage VS.

Sixth Preferred Embodiment

FIG. 14 is a cross-sectional view showing the structure of asemiconductor device according to a sixth preferred embodiment of theinvention: FIG. 14 is associated with FIG. 5. An n⁺-type impurity region66 is formed in contact with the p⁺-type impurity region 33, and ap⁺-type impurity region 65 is formed in contact with the n⁺-typeimpurity region 32. The p⁺-type impurity region 33 and n⁺-type impurityregion 66 are formed in the p-type well 29. The p⁺-type impurity region33 and n⁺-type impurity region 66 are in contact with an electrode 68and the electrode 68 is connected to the high-voltage-side floatingoffset voltage VS. The n⁺-type impurity region 32 and p⁺-type impurityregion 65 are in contact with an electrode 67 and the electrode 67 isconnected to the high-voltage-side floating supply absolute voltage VB.

As in the example of FIG. 6, the p⁺-type impurity region 33 and n⁺-typeimpurity region 66, and the n⁺-type impurity region 32 and p⁺-typeimpurity region 65, are formed between the opposite sides of the NMOS 14and PMOS 15. Alternatively, as in the example of FIG. 7, the p⁺-typeimpurity region 33 and n⁺-type impurity region 66 are formed along theperimeter of the p-type well 29, and the n⁺-type impurity region 32 andp⁺-type impurity region 65 are formed between the p⁺-type isolationregion 22 and the p-type well 29 and surround the p-type well 29.

In the p⁺-type impurity region 33 that is in contact with the n⁺-typeimpurity region 66, holes absorbed by the p⁺-type impurity region 33 caninstantaneously recombine with electrons in the n⁺-type impurity region66 and disappear. Also, in the n⁺-type impurity region 32 that is incontact with the p⁺-type impurity region 65, electrons absorbed by then⁺-type impurity region 32 can instantaneously recombine with holes inthe p⁺-type impurity region 65 and disappear. Thus, the efficiency ofabsorption of holes by the p⁺-type impurity region 33 and the efficiencyof absorption of electrons by the n⁺-type impurity region 32 areimproved as compared with the structure shown in FIG. 5.

While the above description has shown an application of the sixthpreferred embodiment to the first preferred embodiment, the sixthpreferred embodiment is applicable also to all other preferredembodiments.

Seventh Preferred Embodiment

FIG. 15 is a cross-sectional view showing the structure of asemiconductor device according to a seventh preferred embodiment of theinvention: FIG. 15 is associated with FIG. 5. Between the NMOS 14 andPMOS 15, a p⁺-type impurity region 71 is formed in the upper surface ofthe n-type impurity region 28. Between the p⁺-type impurity region 71and the NMOS 14, an n⁺-type impurity region 72 is formed in contact withthe p⁺-type impurity region 71. Between the p⁺-type impurity region 71and the PMOS 15, an n⁺-type impurity region 70 is formed in contact withthe p⁺-type impurity region 71.

A trench 75 is formed in the upper surface of the n-type impurity region28 and extends through the p⁺-type impurity region 71; the inside of thetrench 75 is filled with silicon oxide film 73 and polysilicon 74. Also,a p⁺-type impurity region 76 is formed in the portion of the n-typeimpurity region 28 that defines wall surfaces of the trench 75. Thep⁺-type impurity region 76 is in contact with the p⁺-type impurityregion 71.

On the upper surface of the n-type impurity region 28, a floatingelectrode 77 resides in contact with the p⁺-type impurity regions 71 and76. In the example of FIG. 15, the floating electrode 77 is in contactwith the n⁺-type impurity region 72, without being in contact with then⁺-type impurity region 70. However, note that the floating electrode 77may be in contact with the n⁺-type impurity region 70 without being incontact with the n⁺-type impurity region 72, or may be in contact withboth of the n⁺-type impurity regions 70 and 72.

As in the example of FIG. 6, the p⁺-type impurity regions 71 and 76 andn⁺-type impurity regions 70 and 72 are formed between the opposite sidesof the NMOS 14 and PMOS 15. Alternatively, as in the example of FIG. 7,the p⁺-type impurity regions 71 and 76 and n⁺-type impurity regions 70and 72 are formed between the p⁺-type isolation region 22 and the p-typewell 29 and surround the p-type well 29.

With the p⁺-type impurity regions 71 and 76 and n⁺-type impurity regions70 and 72 being connected to the floating electrode 77, the PN shorteffect lowers the potential of the p⁺-type impurity regions 71 and 76 tocause them to absorb holes in the vicinity, and increases the potentialof the n⁺-type impurity regions 70 and 72 to cause them to absorbelectrons in the vicinity. Then holes and electrons recombine anddisappear by metal short that has an infinitely great recombinationrate. Thus, holes and electrons that have flowed into the n-typeimpurity region 28 due to negative variation of the high-voltage-sidefloating offset voltage VS are absorbed respectively by the p⁺-typeimpurity regions 71 and 76 and the n⁺-type impurity regions 70 and 72and disappear, whereby latchup breakdown of the CMOS 12 is avoided.

Also, forming the p⁺-type impurity region 76, which is in contact withthe p⁺-type impurity region 71, on the wall surfaces of the trench 75enlarges the effective area of the p⁺-type impurity region 71.Furthermore, the electric field strength at the bottom of the trench 75is higher than the electric field strength at the curvature portion ofthe p-type well 29. Thus, the p⁺-type impurity regions 71 and 76 canabsorb more of the hole current flowing into the n-type impurity region28. It is thus possible to more efficiently avoid latchup breakdown ofthe CMOS 12.

While one of the n⁺-type impurity regions 70 and 72 may be omitted,forming both is more effective.

In the example of FIG. 15, the trench 75 is deeper than the p-type well29. Forming the trench 75 deeper increases the area of the p⁺-typeimpurity region 76 and thus enhances the hole current absorptionefficiency.

However, when the trench 75 is deeper than 20 μm, the breakdown voltageis reduced. Accordingly, when it is necessary to also satisfy thebreakdown voltage characteristic, the reduction in breakdown voltage canbe avoided by setting the depth of the trench 75 at 20 μm or less.

Eighth Preferred Embodiment

In the semiconductor devices of the first to seventh preferredembodiments, energy levels, serving as intentionally controlledrecombination centers, are introduced and utilized to cause accumulatedcarriers to recombine and disappear. For example, crystal defects areformed throughout the depth direction of the p⁻-type silicon substrate21 by electron-beam radiation and annealing, platinum diffusion, or golddiffusion and the crystal defects are utilized as carrier lifetimekiller.

According to the semiconductor device of the eighth preferredembodiment, the carrier lifetime of the parasitic diodes PD1-PD4 shownin FIGS. 4 and 5 is shortened. This reduces the amount of carriersaccumulated in the parasitic diodes PD1-PD4, which reduces the forwardcurrent and suppresses the reverse recovery current of the parasiticdiodes PD1-PD4. This enhances the tolerance for malfunction caused bynegative variations of the high-voltage-side floating offset voltage VS.

Also, forming carrier lifetime killer reduces the hole current flowinginto the n-type impurity region 28 due to negative variation ofhigh-voltage-side floating offset voltage VS, which avoids latchupbreakdown of the CMOS 12.

Ninth Preferred Embodiment

The eighth preferred embodiment forms crystal defects by electron-beamradiation and annealing, platinum diffusion, or gold diffusion. However,since this method forms crystal defects all through the depth directionof the p⁻-type silicon substrate 21, the leakage current of NMOS 14 andPMOS 15 may increase. Also, since electron-beam radiation induces pluscharge in the gate oxide films of the high-breakdown-voltage MOS 11,NMOS 14 and PMOS 15, the gate-source threshold voltage characteristicsmay vary.

Accordingly, in the semiconductor devices of the first to seventhpreferred embodiments, the ninth preferred embodiment forms crystaldefects at a given depth in the p⁻-type silicon substrate 21 (in aposition deeper than the channel regions of high-breakdown-voltage MOS11, NMOS 14 and PMOS 15) by helium radiation and annealing, or protonradiation and annealing, and utilizes the crystal defects as carrierlifetime killer. Helium radiation and proton radiation induce crystaldefects intensively in the vicinity of the range of ions (the depth towhich ions penetrate in silicon). It is thus possible to locally controlthe carrier lifetime in the depth direction of the p⁻-type siliconsubstrate 21.

The semiconductor device of the ninth preferred embodiment provideseffects to avoid the increase in leakage current and the variation inthreshold voltage, in addition to the effects provided by the eighthpreferred embodiment.

Tenth Preferred Embodiment

FIG. 16 is a cross-sectional view showing the structure of asemiconductor device according to a tenth preferred embodiment of theinvention: FIG. 16 is associated with FIG. 5. While the first to ninthpreferred embodiments have described the structure of thehigh-voltage-side driving portion 101, the first to ninth preferredembodiments can be applied also to the low-voltage-side driving portion102 as shown in FIG. 16.

In the low-voltage-side driving portion 102, a thyristor isparasitically formed by the p⁺-type source region 15S, n-type impurityregion 28, p-type well 29, and n⁺-type source region 14S. When a surgevoltage higher than the VCC voltage is applied to the output terminalLO, then holes flow into the n-type impurity region 28 from the p⁺-typedrain region 15D that is connected to the output terminal LO, which holecurrent flows into the p-type well 29. Then the parasitic bipolartransistor formed by the n-type impurity region 28, p-type well 29 andn⁺-type source region 14S operates to possibly cause latchup of theparasitic thyristor.

However, when the first to ninth preferred embodiments are applied, e.g.when p⁺-type impurity regions 33, 55, 64 are formed as shown in FIG. 16,then the p⁺-type impurity regions 55 and 64 absorb the hole currentflowing from the p⁺-type drain region 15D into the n-type impurityregion 28 before it flows into the p-type well 29 and the p⁺-typeimpurity region 33 absorbs the hole current flowing into the p-type well29. Latchup of the parasitic thyristor can thus be avoided.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A semiconductor device for driving a switching device having a firstelectrode, a second electrode, and a control electrode, saidsemiconductor device comprising: a first terminal connected to saidfirst electrode; a second terminal connected to said first electrodethrough a capacitive element; a semiconductor substrate having a firstconductivity type; a first impurity region formed in a main surface ofsaid semiconductor substrate and having a second conductivity type; asecond impurity region formed in a main surface of said first impurityregion and having said first conductivity type; a first transistorhaving a source/drain region of said second conductivity type, saidsource/drain region of said first transistor being formed in a mainsurface of said second impurity region and connected to said firstterminal; a second transistor having a source/drain region of said firstconductivity type, said source/drain region of said second transistorbeing formed in the main surface of said first impurity region andconnected to said second terminal; a third impurity region formed insaid main surface of said first impurity region and having said firstconductivity type; a trench formed in said main surface of said firstimpurity region and extending through said third impurity region; afourth impurity region having said first conductivity type and formed insaid first impurity region in a portion that defines wall surfaces ofsaid trench; a fifth impurity region having said second conductivitytype and formed in said main surface of said first impurity region andin contact with said third impurity region; and a floating electrodeformed on said main surface of said first impurity region and in contactwith said third to fifth impurity regions.
 2. The semiconductor deviceaccording to claim 1, further comprising an isolation region having saidfirst conductivity type and formed in said main surface of saidsemiconductor substrate and in contact with said first impurity region,wherein said third to fifth impurity regions are formed between saidisolation region and said second impurity region and surround saidsecond impurity region continuously or intermittently.
 3. Thesemiconductor device according to claim 1, further comprising a sixthimpurity region formed in said main surface of said first impurityregion and having said second conductivity type, said sixth impurityregion being in contact with said third impurity region on a sideopposite to said fifth impurity region.
 4. The semiconductor deviceaccording to claim 1, wherein said trench is deeper than said secondimpurity region.
 5. A semiconductor device for driving a switchingdevice having a first electrode, a second electrode, and a controlelectrode, said semiconductor device comprising: a first terminalconnected to said first electrode; a second terminal connected to saidfirst electrode through a capacitive element; a first impurity regionhaving a first conductivity type; a second impurity region formed in amain surface of said first impurity region and having a secondconductivity type; a first transistor having a source/drain region ofsaid first conductivity type, said source/drain region of said firsttransistor being formed in a main surface of said second impurity regionand connected to said first terminal; a second transistor having asource/drain region of said second conductivity type, said source/drainregion of said second transistor being formed in the main surface ofsaid first impurity region and connected to said second terminal; and athird impurity region formed in said main surface of said first impurityregion, having said second conductivity type, and connected to saidfirst terminal.